1. Field of the Invention
This invention relates to hydraulic steering systems suitable particularly for large vehicles such as off-the-road construction and earth moving machinery although it is adaptable to vehicles of smaller size.
2. Description of the Prior Art
It is known to utilize two pumps to provide hydraulic fluid for the operation of a vehicle hydraulic steering system and to include an accumulator in the hydraulic circuit to assist in maintaining a relatively constant hydraulic pressure. The use of two pumps minimizes the possibility of failure or malfunction of the hydraulic steering system if one of the pumps fails or ceases to operate for any reason. One of the pumps may supply the hydraulic system normally with the other arranged to be connected actively into the system only when the first pump fails, or it is possible for both of the pumps to supply the hydraulic system jointly under normal conditions with either serving as an emergency pump if the other fails. It is common for one of the pumps to be driven by the engine of the vehicle and the other to be operated responsively to motion of the vehicle, for example, through an output element of the vehicle transmission.
It is known in the prior art to provide for deactivating such a steering system by draining or discharging the accumulator when the vehicle engine is turned off and thus depressurizing the steering system. However, with such an arrangement it is possible to deactivate the steering system and thus make it impossible to steer the vehicle even though the vehicle may be still moving. Moreover, in a hydraulic system embodying an accumulator the hydraulic pressure necessary to actuate the steering mechanism may be retained for a considerable time after the pump or pumps are stopped, and this is undesirable because it would allow the steering mechanism to be moved after operation of the vehicle had ceased and when maintenance or other activities were taking place with the resultant possibility of personal injury or property damage because of movement of parts of the vehicle. The object of the present invention is to overcome these deficiencies.